1. Field of the Invention
The present invention relates to methods and apparatus for producing ultra-fine fibers and ultra-fine webs of fibers utilizing a spunbond process.
2. Description of the Related Art
The spunbond process, a direct one-step method to manufacture fabric from polymer materials utilizing a spin and bond method, was first commercialized by DuPont Corporation in 1959 with the formation of a polyester nonwoven product sold under the trademark REEMAY®. In the half century since much progress has been made in the spunbond process, with many different products available based upon the selection of one or more polymers to be used in the process. The global growth rate for spunbond products has increased considerably over this period of time, higher than any other nonwoven technology, and suppliers of medical and hygiene products have switched almost completely to spunbond or spunbond composites.
The fiber fineness or size produced in a spunbond process is typically greater than about 1.0 denier, despite the efforts of spunbond developers to produce sub-denier products economically. The term “denier” refers to the mass in grams per 9,000 meters of fiber. In particular, it is presently very difficult to obtain spunbond fabrics having a fineness in the range of about 0.5 dpf (denier per fiber) or less due to production, economic, and various technical factors associated with spunbond processes.
To obtain the benefits of finer fibers, and smaller pore size for nonwoven fabrics formed with such fibers, manufacturers have resorted to using meltblown processes to form fibers with smaller dimensions for use in manufacturing fabrics. Generally, a meltblown process differs from a spunbond process in that extruded polymer filaments, upon emerging from an extruder die, are immediately blown with a high velocity, heated medium (e.g., air) onto a suitable support surface. In contrast, extruded but substantially solidified filaments (e.g., utilizing a suitable quenching medium such as air) in a spunbond process are drawn and attenuated utilizing a suitable drawing unit (e.g., an aspirator or godet rolls) prior to being laid down on a support surface. Meltblown processes are typically utilized in forming fibers having diameters on a micron level, whereas spunbond processes are typically employed to produce fibers having normal textile dimensions.
To date, manufacturers have produced laminates including three or more nonwoven layers, where a layer of meltblown microfibers (including fibers with average diameters or average cross-sectional dimensions in the range of 2-4 micrometers or microns) is sandwiched between two layers of macrofiber spunbond products. An example of such a laminate is described in U.S. Pat. No. 4,810,571, the disclosure of which is incorporated herein by reference in its entirety. Laminates such as these are referred to as “SMS” laminates (i.e., referring generally to any combination of one or more meltblown layers sandwiched between two or more spunbond layers, such as spunbond-meltblown-spunbond, spunbond-meltblown-meltblown-spunbond, spunbond-spunbond-meltblown-spunbond-spunbond, etc.). The meltblown layer must be sandwiched between spunbond layers, since the tenacity of meltblown fibers is not very large in comparison to spunbond fibers.
From a performance standpoint, SMS laminates have performed better than traditional spunbond fabrics and are satisfactory in certain applications. However the investment cost to produce such laminates is quite high due to the requirement of having spunbond layers surrounding meltblown layers. In addition, the meltblown portion of the fabric has low orientation with resulting low tensile properties. The meltblown layer can also be relatively amorphous depending on the polymer used to form the meltblown fibers. Further, the size distribution of meltblown fibers is significantly broad, such that meltblown fabric layers often include a significant percentage of larger fibers having diameter dimensions that are 100% or greater in comparison to the average fiber dimensions of the fabric.
Fabric performance could be enhanced, particularly in areas such as filtration, fabric drape, softness, and coverage, if fabrics could be formed with fibers as fine or finer than the meltblown fibers that are substantially uniform in cross-sectional dimensions and have tensile and crystalline properties of spunbond fibers.
Another problem in spunbond processes that produce complex plural component fibers (e.g., bicomponent fibers) is that it has been necessary to arrange multiple small spin packs and drawing units together in a direction transverse the web laydown and travel direction in order to achieve a resultant nonwoven fabric from the drawn fibers that is at least of sufficient width (e.g., 500 millimeters or greater in width). This in turn contributes to problems in uniformity of the fabric laydown.
A further problem for both spunbond and meltblown processes is the difficulty in producing hollow or tubular nanofibers of sufficient dimensions (e.g., between about 500 nanometers or less in diameter). In particular, it is desirable to produce carbon nanofibers from an extrusion process for a variety of different applications. Carbon fibers are lightweight and have extremely high strength characteristics that make them useful in forming a number of different products, such as fishing rods, tennis rackets shafts for golf clubs, rigid components for automobiles and aircraft, etc. In addition, hollow carbon nanofibers hold great promise for use in engineering and medical devices such as artificial kidneys and other organ transplants, microfiltration devices, etc.
It is known to manufacture carbon nanofibers by extruding melt processable polyacrylonitrile (PAN) in a spunbond or meltblown process, followed by subjecting the extruded PAN fibers to a carbonization process to form carbon fibers. One example of such a process is described in U.S. Pat. No. 6,583,075, which is incorporated herein by reference in its entirety. In particular, the '075 patent describes the formation of multicomponent fibers (e.g., pie/wedge fibers, islands-in-the-sea fibers, etc.), in which one component is PAN and the other component is dissolvable from PAN, such that PAN microfibers can be formed from the multicomponent fiber, and the PAN microfibers are then converted to graphite fibers in a carbonization process.
While processes have been developed to form extruded PAN microfibers that can be converted to carbon microfibers, difficulties still exist in attempting to form an extruded hollow PAN tube on the order of micron or even nanometer diameter dimensions. This is due, in part, to the difficulty associated with extruding a hollow fiber on the micron or nanometer diameter dimensions without having collapsing or deforming, either by the surface tension of the solidifying fiber or the tension applied to the fiber, after extrusion. In addition, typical extrusion processes simply cannot achieve sufficient productivity levels for generating hollow microfibers that renders the process efficient and economical. Accordingly, a need exists to reliably and efficiently manufacture hollow PAN tubular fibers on micron or nanometer dimensions that can then be converted to carbon tubes.